1. Field of the Invention
The present invention relates generally to the art of gyrocompasses and more particularly to remotely located heading display repeater devices operated from a gyrocompass step-by-step data transmitter.
2. Description of the Prior Art
Prior art step-by-step heading repeater devices utilize analog data receivers in which a repeater motor mechanically drives an indicating compass card through gearing corresponding to that present in the step-by-step transmitter. Craft heading in degrees is obtained from an angularly calibrated scale printed on the rotatable compass card by observations with respect to a lubber line marked on the stationary frame of the repeater. Several such repeaters may be disposed conveniently about the craft for navigation and other purposes.
The repeater-driving electrical signals of the data transmitter are step-by-step data outputs that are normally applied through amplifier channels to three pairs of field poles in the repeater motor. In a typical step-by-step transmitter, which is mounted on the conventional lubber ring of the gyrocompass, there are brushes that roll over commutation segments and which, when electrical contact is made, supply current to respective pairs of poles of the compass repeater motor. The soft iron armature of the repeater motor then orients itself between one pair of its poles, then half way between that pair and the next, and so on. This angular stepping of the repeater motor armature turns the compass card, thereby incrementally following the gyrocompass heading. The invention is not limited to use with such data transmitters and may be used, for example, with known data transmitters employing light emitting diodes, photoresistors, and a rotatable vaned shutter.
Because of the generally hostile nature of shipboard environments and of the susceptibility of semiconductor elements to overload damage, short-circuit protection of the repeater channels is required in the interest of achieving acceptable reliability. Prior practice has seen general use of various current-limiting techniques for this protection, but with the achievement of only limited reliability. Automatic reset is not feasible and fuses or circuit breakers are normally required. Momentary or intermittent fault conditions are not detected and, even if detected, are difficult to locate. As the power output level of the amplifier channels is desirably increased for operating pluralities of repeaters, the current-limiter technique becomes increasingly undesirable. In particular, the problem with the prior art current limiting technique lies in the level of power dissipation in the output semiconductor devices. At higher power levels, the failure rates of these devices become excessive in the current-limiting mode due to junction over-temperature. The present invention overcomes this problem by limiting the durations of such power dissipation to very short time periods.